The present application relates to fluid jet print heads and, more particularly, to a print head having a stimulation arrangement of the type which produces pressure varicosities in the individual fluid jets, resulting in substantially uniform breakup of the jets into streams of drops.
Ink jet printers, incorporating fluid jet print heads, are known which have an orifice structure defining a plurality of orifices. The orifices receive an electrically conductive recording fluid, such as a water-base ink, from a pressurized fluid supply manifold and eject the fluid in one or more rows of parallel streams. As the streams break up into drops, the drops are selectively charged and deflected, with some of the drops being deposited on a print-receiving medium and the balance of the drops being caught by an appropriate catcher structure.
Charging of the drops is accomplished by selectively applying charging voltages to charge electrodes positioned near each of the streams. The fluid flowing through each orifice emerges as a fluid filament. Drops break away from the tip of the fluid filament and carry charges related to the voltage of the associated charge electrode at the instant of drop formation. Each drop is then subjected to an electrostatic field which deflects the drop by a distance proportional to the magnitude of the charge which it carries. Drops may thus be deflected to one or more print positions or, when a drop is not to be deposited on the print-receiving medium, deflected to an adjacent catcher structure.
With print heads of the type used in ink jet printers, it is necessary to control drop formation since if left to natural stiumulating disturbances, the fluid filaments would break up erratically into drops of various sizes at irregular intervals. Such erratic drop formation would prevent proper charging and deflection of the drops. Accordingly, it is customary to apply a stimulating disturbance to all of the fluid streams to produce jets of uniformly sized and regularly spaced drops.
Various types of stimulation arrangements have been suggested. U.S. Pat. No. 3,739,393, issued June 12, 1973, to Lyon et al, discloses an ink jet print head in which the fluid orifices are defined by a thin, relatively flexible orifice plate. A piezoelectric transducer contacts the orifice plate at one end and produces a series of bending waves which travel longitudinally along the plate. Dampers at each end of the orifice plate dampen these traveling waves and prevent wave reflection. The bending waves in the orifice plate produce an oscillatory movement of the orifices which, in turn, causes pressure varicostities in the fluid filaments energing from the orifices. As a consequence, the fluid filaments break up into relatively uniform jet drop streams.
It will be appreciated that break up of the drop streams is nonsynchronous in a print head employing traveling wave stimulation. The print head, therefore, cannot be operated at its maximum printing resolution since the precise time of drop formation for each stream will be unknown and charge voltages must be supplied to the charge electrodes for sufficient time periods to insure that they result in appropriate charging of at least one drop. As a consequence more than one drop is usually charged in succession and partially charged drops, formed during charge voltage transition periods, are commonly formed.
One solution to this problem is to apply drop stimulating disturbances to all filaments in synchronism. If all of the jets have the same diameter and velocity, and stimulating disturbances are applied to the jets simultaneously, all filaments will generate drops in synchronism. Such synchronized drop generation greatly simplifies the application of charge signals to the charge electrodes, because the timing for each of the jet transitions can be timed to occur between drop formations. The number of partially charged drops is therefore substantially reduced.
One print head that employs this approach to drop stimulation is disclosed in copending, commonly assigned U.S. patent application Ser. No. 496,159 filed May 19, 1983 now abandoned and refiled as C-I-P Ser. No. 630,926, filed July 16, 1984. The print head includes a manifold defining an elongated cavity and an orifice plate defining a plurality of orifices arranged in a single row. A transducer arrangement is mounted in the cavity, spaced from the orifices plates so as to define a fluid reservoir therebetween. The transducer, when electrically excited, produces pressure waves of substantially uniform wave front which travel through the fluid in the reservoir toward the orifice plate and cause breakup into jet drop steams of fluid flowing through the orifices. Since the pressure waves are in the form of a wave front, the amplitude and phase of the fluid stimulation at each orifice is substantially uniform across the orifice plate.
The transducer is formed from an elongated block of piezoelectric material which has a plurality of slots defined therein extending alternately from opposite sides of the transducer partially therethrough. Each slot is substantially perpendicular to the row of orifices, and prevent wave propagation along the transducer. Additionally, the transducer is mounted within acoustic isolation material to prevent unwanted vibrations from being transmitted through the cavity walls.
Notwithstanding the foregoing preventative measures, it has been found that it is not practically possible to produce a wave front that is exactly uniform in amplitude and phase along its length. While such a stimulation arrangement yields better performance than the traveling wave approach of, for example, Lyon et al, any variation in the frequency or amplitude with which one or more portions of the transducer operates can cause traveling waves to be produced which move laterally along the fluid reservoir. This in turn causes non-synchronous drop generation.
What is needed, therefore, is a print head arrangement utilizing wave front stimulation that will further improve such an approach to drop formation with respect to the traveling wave method by preventing the formation of standing waves within the fluid reservoir.